In a passive infrared intrusion detection system, infrared radiation in a space to be protected is sensed to produce an electrical signal which represents the level of received radiation in the protected area. The presence of an intruder in the protected area gives rise to an increase in the radiation level which is sensed to provide an output indication of intruder presence when the level of received radiation exceeds a predetermined threshold level. Examples of passive infrared intrusion detection systems are shown in U.S. Pat. Nos. 3,036,219; 3,524,180; 3,631,434; 3,703,718 and 3,886,360.